1. Field of the Invention
The present invention relates generally to a magnetic field measuring apparatus. In particular, the present invention relates to an apparatus and method for measuring a second-order gradient of a magnetic field using a super conductor quantum interference device (SQUID) which obtains the second-order gradient of the magnetic field with a simple-constructed and low-priced measuring apparatus having three SQUID sensors and one differential circuit only.
2. Description of the Related Art
The SQUID is an element which can respond to the change of a weak magnetic field based on the quantum interference effect of flux, and is used in a flux meter or a biosensor.
However, the SQUID has the problems in that since the source of a magnetic field is in the form of a dipole, the attenuation of the magnetic field according to a distance becomes severe, and it becomes more severe especially in case that the size of the magnetic field is smaller than that of the surrounding environment such as biomagnetism.
In order to solve the problems of the magnetic field attenuation, a sensor is brought close to a signal source, and a gradient type signal, which is a differential value for a space, is measured by the sensor. This measurement is technically simple, and advantageous in sensing the change of the signal source according to a time in a local space.
As shown in FIG. 1a, a basic DC SQUID, which is a device for measuring a magnetic field, comprises a super conductor loop C that has a small inductance and includes two super conductor junctions A and B.
The electromagnetic characteristic of the DC SQUID is that when a DC bias current Io flows through the two super conductor junctions A and B, the voltage VJ of the super conductor junctions A and B is changed according to the flux "PHgr"a passing through the loop C of the SQUID as shown in FIG. 1b. 
For instance, if the voltage of the super conductor junction A is VA, and the voltage of the super conductor junction B is VB, the changed voltage VJ will be VAxe2x88x92VB. Also, the voltage VJ appears as a function of flux that oscillates with a period of one flux quantum "PHgr"a as shown in FIG. 1c. 
Most apparatuses for measuring a magnetic field using the SQUID as described above includes a flux locked loop, which is a feedback section for maintaining the amount of flux passing through the SQUID loop to be always constant.
The conventional magnetic field measuring apparatus using the SQUID is briefly classified into three methods.
The first method connects to the SQUID and measures the current picked up according to the difference between magnetic fields produced by two space-apart pickup coils when the magnetic fields are produced from the two coils, respectively. The second method converts outputs of two magnetic sensors into electric signals or digital signals, and subtracts one converted output value from the other. The third method is an active compensation method for artificially forming a magnetic field which is opposite to the magnetic field exerted on a reference magnetic sensor with respect to other magnetic sensors based on a signal obtained from an output of the reference magnetic sensor, and sensing only the difference therebetween.
According to the third method, the offset of the magnetic field is primarily performed by reading out the magnetic field of the reference sensor from the magnetic field surrounding the SQUID and applying to other sensors a magnetic field which has the opposite direction to and the same size as the magnetic field of the reference sensor, and thus it is not required to heighten the dynamic range with respect to other circuits except the reference sensor.
As described above, an effective measuring apparatus can be produced by employing the third method, which is used for measuring the first-order gradient using three sensors including one reference sensor and two differential sensors.
However, according to the first method, though the measurement of the magnetic field is possible by using low-noise pickup coils in case of a low-temperature SQUID, it is technically difficult to make desired types of pickup coils since the pickup coils of a good quality can be made in the form of a thin film in case of a high-temperature SQUID.
According to the second method, the ratio of the dynamic range to sensitivity of a respective readout circuit, i.e., the signal-to-noise (S/N) ratio, should become great in order to sufficiently use the sensitivity in the general environment. This S/N ratio should have the size of 23 bits which corresponds to more than 140 dB since the magnetic field strength in the general environment is about 10xe2x88x926T, and the sensitivity of the general high-temperature SQUID is about 10xe2x88x9213T.
It is technically difficult to implement the readout circuits having a very large S/N ratio as above. Also, the common mode rejection ratio (CMRR) of the differential circuit should become great in proportion to the S/N ratio of the readout circuit, but the CMRR of the differential amplifier is limited in case that a large DC magnetic field is commonly exerted on the respective sensors.
Also, in order to measure the second-order gradient by the third method, it is not required for the three driving circuits to have a high S/N ratio as in the second method, but more than three differential amplifiers and associated digital calculation are still required as well as 7 coils.
Also, according to the third method, the M (proportional constant of the current applied to coils and the magnetic field produced by the current) value of the three coils with respect to the reference sensor should be adjusted, and two among the three circuits should be re-adjusted to equally correcting the driving circuits of the three sensors, thereby requiring a number of adjustment steps.
Accordingly, the present invention is directed to an apparatus and method for measuring a second-order gradient of a magnetic field using a SQUID that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an apparatus and method for measuring a second-order gradient of a magnetic field using a SQUID which obtains the second-order gradient of the magnetic field with a simple-constructed and low-priced magnetic field measuring apparatus that can be used in the general environment and has three SQUID sensors and one differential circuit only.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the apparatus for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention includes first to third SQUID sensor driving circuits for detecting respective surrounding magnetic fields and outputting corresponding currents, respectively, a first coil for producing a magnetic field according to the current outputted from the first SQUID sensor driving circuit and feeding the produced magnetic field back to the first SQUID sensor driving circuit, a second coil for producing a magnetic field according to a sum of the currents outputted from the first and second SQUID sensor driving circuits and feeding the produced magnetic field back to the second SQUID sensor driving circuit, a third coil for producing a magnetic field according to a sum of the currents outputted from the second and third SQUID sensor driving circuits and feeding the produced magnetic field back to the third SQUID sensor driving circuit, and a differential amplifying section for differential-amplifying voltages outputted from the second and third SQUID sensor driving circuits.
In another aspect of the apparatus for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention, the respective SQUID sensor driving circuits are connected to the first to third coils, respectively, and one of the SQUID sensor driving circuits acts as a reference sensor, while the remaining two thereof act as differential sensors.
In still another aspect of the apparatus for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention, first and second bypass variable resistors are connected in parallel to the second and third coils, respectively.
In still another aspect of the apparatus for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention, the first to third coils are connected in series.
In still another aspect of the apparatus for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention, the reference sensor is located at the first or last position of the sensors.
In still another aspect of the present invention, there is provided a method for measuring a second-order gradient of a magnetic field using a SQUID comprising the steps of positioning first to third SQUID sensors which are driven in a feedback manner at equal intervals, connecting in series a first feedback coil connected to the first SQUID sensor, a second feedback coil the first feedback coil and a current applied to the second feedback coil and accumulating the added current in the second feedback coil, adding a current applied to the second feedback coil and a current applied to the third feedback coil and accumulating the added current in the third feedback coil, producing magnetic fields according to the accumulated currents in the first to third feedback coils and feeding the produced magnetic fields back to the first to third SQUID sensors, and obtaining a second-order gradient value using a difference between voltages produced by the second and third SQUID sensors.
In another aspect of the method for measuring a second-order gradient of a magnetic field using a SQUID according to the present invention, first and second bypass variable resistors are connected in parallel to the second and third feedback coils, respectively, and a magnetic field which is the same as the magnetic field produced in the first feedback coil is produced in the second and third feedback coils by adjusting the first and second variable resistors.